In electrography, an electrostatic charge image is formed on a dielectric surface, typically the surface of a photoconductive recording element or photoconductor. Development of this image is commonly achieved by contacting it with a dry, two-component developer comprising a mixture of pigmented resinous electrically insulative particles known as toner, and magnetically attractable particles, known as carrier. The carrier particles serve as sites against which the non-magnetic toner particles can impinge and thereby acquire a triboelectric charge opposite to that of the electrostatic image. The toner particles are held on the surface of the relatively larger-sized carrier particles by the electric force generated by the friction of both particles as they impinge upon and contact one another during mixing interactions. During contact between the electrostatic image and the developer mixture, the toner particles are stripped away from the carrier particles to which they had formerly adhered (via triboelectric forces) by the relatively strong attractive force of the electric field formed by the charge image which overcomes the bonding forces between the toner particles and the carrier particles. In this manner, the toner particles are attracted by the electrostatic forces associated with the charge image and deposited on the electrostatic image to render it visible.
It is known in the art to apply developer compositions of the above type to electrostatic images by means of a rotating-core magnetic applicator which comprises a cylindrical developing sleeve or shell of a non-magnetic material having a magnetic core positioned within. This particular type of development is commonly referred to in the art as magnetic brush development. The core usually comprises a plurality of parallel magnetic strips which are arranged around the core surface to present alternative north and south magnetic fields. These fields project radially, through the sleeve, and serve to attract the developer composition to the sleeve's outer surface to form a brush nap, or what is commonly referred to in the art as, a "magnetic brush." It is essential that the magnetic core be rotated during use to cause the developer to advance from a supply sump to a position in which it contacts the electrostatic image to be developed. The cylindrical sleeve, or shell, may or may not also rotate. If the shell does rotate, it can do so either in the same direction as or in a different direction from the core. After development, the toner depleted carrier particles are returned to the sump for toner replenishment. The role of the carrier is (a) to transport the toner particles from the sump to the magnetic brush, (b) to charge the toner by triboelectrification to the desired polarity, i.e., a polarity opposite that of the charge of the electrostatic image on the photoconductive recording element or plate and (c) to charge the toner to the proper or desired degree (amount) of charge. The magnetic carrier particles, under the influence of the magnets in the core of the applicator, form fur-like hairs or chains extending from the developing sleeve or shell of the applicator. Since the charge polarity of the magnetic carrier is the same as that of the electrostatic image, the magnetic carrier is left on the developing sleeve of the applicator after the toner particles have been stripped away from the carrier during development of the electrostatic or charge image. Typically, a bias voltage is applied between the photosensitive material or plate and the developing sleeve of the magnetic applicator by means of an electric current externally applied to the developing sleeve or shell which flows through the magnetic brush. The purpose of the bias voltage primarily is to prevent, or at least substantially reduce, the occurrence of unwanted toner fogging or background development caused by the migration of a certain portion of the toner particles available for development from the carrier to a non-image area or portion of the photosensitive plate (or drum) during development due to an incomplete discharge of such non-image areas during exposure. Commonly referred to as background charge, these areas off incomplete discharge cause an attraction for and a migration of some of the available toner particles (particularly those toner particles possessing an insufficient quantity of charge) to the partially discharged areas during development which results in the development or coloration of areas of the electrostatic image pattern that should not be developed. The polarity of the bias voltage should be the same as the charge polarity of the photosensitive material. That is, if the charge polarity of the photosensitive material or plate is positive, a positive polarity is selected for the bias voltage. Caution must be exercised in selecting the proper amount of bias voltage applied between the photosensitive material and the developing sleeve so that problems such as discharge breakdown are not caused in the photosensitive material or the magnetic brush or that toner migration of the toner particles from the carrier to the electrostatic image to be developed is not prevented due to the application of a disproportionate or excessive amount of bias voltage to the magnetic brush during development. Ordinarily, it is typical that the bias voltage be controlled to about 25 to 300 volts, particularly about 150 to 250 volts.
Conventionality, carrier particles made of soft magnetic materials have been employed to carry and deliver the toner particles to the electrostatic image. More recently, hard magnetic materials have been used to carry and deliver the toner particles to the electrostatic image. For example, U.S. Pat. Nos. 4,546,060 to Miskinis et al, and 4,473,029 to Fritz et al, teach the use of hard magnetic materials as carrier particles and an apparatus for the development of electrostatic images utilizing such hard magnetic carrier particles, respectively. These patents require that the carrier particles comprise a hard magnetic material exhibiting a coercivity of at least 300 Oersteds when magnetically saturated and an induced magnetic moment of at least 20 EMU/g when in an applied magnetic field of 1000 Oersteds. The terms "hard" and "soft" when referring to magnetic materials have the generally accepted meaning as indicated on page 18 of Introduction to Magnetic Materials by B. D. Cullity published by Addison-Wesley Publishing Company, 1972. These hard magnetic carrier materials represent a great advance over the use of soft magnetic carrier materials in that the speed of development is remarkably increased without experiencing deterioration of the image. Speeds as high as four times the maximum speed utilized in the use of soft magnetic carrier particles have been demonstrated.
The above two mentioned U.S. patents, while generic to all hard magnetic materials having the properties set forth, prefer the hard magnetic ferrites which are compounds of barium and/or strontium such as, BaFe.sub.12 O.sub.19 and SrFe.sub.12 O.sub.19. While these hard ferrite carrier materials provide for increased development speeds, it is desired that even further improvements be made with respect to further increasing development speed using these ferrite carrier materials. It is toward this objective that the present invention is directed.
It has now been discovered that by utilizing a mixture of these particular ferrite carrier particles in which from 1.0 to 10.0% by weight of the total weight of the mixture comprises carrier particles having a number average particle diameter of from 1.0 to 10.0 micrometers and from 99.0 to 90.0% by weight of the total weight of the mixture comprises carrier particles having a number average particle diameter of from 11.0 to 38.0 micrometers, that developer compositions comprising such carrier particles and oppositely charged toner particles exhibit development speeds of from 1.5 to 2.5 times faster than those of conventional developer compositions comprising carrier particles having a typical particle size distribution of from about 1.0 to about 60.0 micrometers and oppositely charged toner particles. That is, in the conventional carrier manufacturing process for producing strontium and barium ferrite carrier particles, powders of ferric oxide (i.e., Fe.sub.2 O.sub.3) and the oxides of barium or strontium or a salt of barium or strontium convertible to the oxide by heat, such as the carbonates, sulfates, nitrates or phosphates of barium or strontium, are mixed together in a predetermined ratio, typically from about 4 to 6 moles of Fe.sub.2 O.sub.3 per 1 mole of the metal oxide or metal oxide-forming salt and then mixed with a solution of an organic binder, such as guar gum, and a polar solvent, preferably water. The solution is then ball milled into a liquid slurry and spray dried to form unreacted, non-magnetic, dried green beads. The green beads are then subsequently fired at high temperatures, generally ranging from about 900.degree. to 1500.degree. C. to form the magnetic carrier particles typically having a number average particle size distribution of from about 1.0 to about 100.0 micrometers, and more typically a number average particle size distribution of from about 1.0 to about 60.0 micrometers. This particular method of carrier particle manufacture is commonly referred to as the spray-drying method of manufacture. It has been found, however, that instead of forming developer compositions by simply mixing the carrier particles as they are obtained from the spray drying process having a particle size distribution of from about 1.0 to about 60.0 micrometers (number average particle diameter) with oppositely charged toner particles and using such developer compositions to develop electrostatic images in a copying apparatus, that if that fraction of the carrier particles produced by the spray drying process having a number average particle diameter of from 11.0 to 38.0 micrometers and that fraction of the carrier particles having a number average particle diameter of from 1.0 to 10.0 micrometers are separated out and combined or blended together, to form a mixture of such carrier particles in which the amount of those carrier particles having a number average particle diameter of from 11.0 to 38.0 micrometers constitutes from 99.0% to 90.0% by weight of the mixture, based on the total weight of the mixture, and the amount of those carrier particles in the mixture having a number average particle diameter of from 1.0 to 10.0 micrometers constitutes from 1.0% to 10.0% by weight of the mixture, based on the total weight of the mixture, and this mixture is used in forming a developer composition, that such developer compositions can increase development speed by as much as 1.5 to 2.5 times over that of a conventional developer composition comprising the same carrier particles having a typical particle size distribution of from about 1.0 to about 60.0 micrometers and the same toner particles having the same composition and electrical charge volume.